I routinely submitted works less than 80% of the required length and still received A grades. Most instructors are reasonable enough to know that high quality work does not need to meet arbitrary constraints.
That works great in cases where the instructors are allowed to think for themselves, not so well otherwise. In Florida we had this law called the Gordon Rule which states that all students are required to write 24,000 words worth of essay text over the course of their college education. They don't literally track how many words students write on an individual basis, but rather designate certain courses as 2,000, 4,000, or 6,000 word courses. Professors are then held strictly accountable for truth-in-advertising. They're required to check every student's word-count, and to flunk any student who goes one word under the quota.
The Gordon rule is not nearly that restrictive. Gordon rule courses re structured so that students will have a lot of writing assignments. No one is getting flunked for writing 5,999 words. The idea is to force students to take courses that require a lot of writing.
The rule is actually not a terrible idea in the first place. Its basically a response to a larger than you'd expect number of florida university graduates not being able to pass the CLAST (a re-hashing of the SAT). There's a similar requirement that all students must take a certain number of math classses.
No one is getting flunked for writing 5,999 words.
I graduated from the University of Florida. YMMV, but my instructors in Gordon Rule courses were quite diligent/paranoid about making sure students met the word quota. I never heard of a student actually getting flunked for being slightly under, but that's because the instructors made sure that they didn't turn in an under-quota paper in the first place -- even if it meant just padding the extra space with repetition of "Gordon Rule sucks." (Ok, so you couldn't be quite that blatant, but not far off)
Edit: UF was in the middle of a reaccreditation audit while I was there. Maybe the situation is less bad the rest of the time.
Administrative fiat, I assume. IIRC all courses that offered Gordon Rule credit were part of the statewide curriculum, not invented by an individual professor. In fact, for that reason I had to double-check my schedule to make sure I was satisfying the Gordon Rule math requirement, despite being a math major. It was never actually a problem, but it's rather broken that it was even a close call.
That works great in cases where the instructors are allowed to think for themselves, not so well otherwise. In Florida we had this law called the Gordon Rule which states that all students are required to write 24,000 words worth of essay text over the course of their college education. They don't literally track how many words students write on an individual basis, but rather designate certain courses as 2,000, 4,000, or 6,000 word courses. Professors are then held strictly accountable for truth-in-advertising. They're required to check every student's word-count, and to flunk any student who goes one word under the quota.